Random orientation of staple in slurry-cast propellants

ABSTRACT

A METHOD FOR RANDOMLY ORIENTING METALLIC STAPLE IN SOLID PROPELLANTS PREPARED BY SLURRY CASTING IS PROVIDED. THE METALLIC STAPLE IS RANDOMIZED BY CASTING POWDER GRANULES WHICH FORM A PART OF THE SLURRY. THE CASTING POWDER GRANULES ARE ADDED IN A SUFFICIENT QUANTITY TO FORM INTERSTICES BETWEEN THE CASTING POWDER GRANULES. THE STAPLE IS TRAPPED IN THESE INTERSTICS DURING MIXING AND THEREBY RANDOMLY ORIENTED.

United States Patent Oflice 3,813,458 RANDOM ORIENTATION OF STAPLE IN SLURRY-CAST PROPELLANTS Ronald L. Simmons, Cumberland, Md., assignor to Hercules Incorporated, Wilmington, Del.

N Drawing. Filed June 5, 1968, Ser. No. 736,930 Int. Cl. C06b 21/02 US. Cl. 264-3 C 11 Claims ABSTRACT OF THE DISCLOSURE A method for randomly orientingfimetallic staple in solid propellants prepared by slurry casting is provided. The metallic staple is randomized by casting powder granules which form a part of the slurry. The casting powder granules are added in a sufiicient quantity to form interstices between the casting powder granules. The staple is trapped in these interstices during mixing and thereby randomly oriented.

This invention relates to a method of randomly orienting metallic staple in slurry-cast propellants.

Metallic staple consists of thin pieces of metal wire or strip and is employed in solid propellants to augment the burning rate of the propellant. It is known that the degree of burning rate augmentation resulting from the staple is dependent on the orientation of the staple in relationship to the burning front of the propellant. For example, maximum burning rate augmentation is achieved when the longitudinal axis of the staple is perpendicular to the burning front of the solid propellant. It is thus critical that the orientation of staple be uniform throughout a solid propellant so that uniform and reproducible ballistic results can be obtained. One method of obtaining a solid propellant containing metallic staple in which uniform and reproducible ballistic results are achieved is to prepare the propellant from casting powder granules containing metallic staple. The random orientation of the granules in a cast propellant charge supplies random orientation of the staple contained within the granules.

Such random patterns have not been achievable thus far in propellant prepared by slurry casting. This results from clustering of the staple during mixing of the slurry and more particularly is due to the tendency of the staple to orient in the direction of propellant flow. Heretofore, there has been no satisfactory method to control this orientation in slurry so that a random orientation of the staple results.

The method of the present invention makes it possible to randomly orient staple in propellants prepared by slurry casting. This method eliminates the need for preparing casting powder granules containing metallic staple which is a costly and hazardous process. The propellant prepared by the method of this invention is substantially cheaper and safer to prepare than conventionally cast solid propellants containing metallic staple.

Accordingly, it is an object of this invention to provide a method for randomly orienting metallic staple in a solid propellant prepared by a slurry casting process.

It is another object of this invention to provide an economical method for preparing a solid propellant containing metallic staple which has a uniform and reproducible burning rate.

It is another object of this invention to provide a comparatively safe method for preparation of high energy solid propellants containing randomly oriented metallic staple.

Other objects of this invention will, in part, be obvious and will in part, appear hereinafter. For a complete understanding of the nature and objects of this invention, reference is made to the following detailed description.

Patented May 28, 1974 Broadly, in accordance with this invention, a method is provided for randomly orienting metallic staple in propellant prepared by a slurry casting process. This method comprises as a first step preparing a propellant slurry containing metallic staple and casting powder granules, said casting powder granules being present in an amount sufficient to provide interstices between the granules throughout the propellant slurry for trapping the metallic staple. The propellant slurry is mixed whereby the staple is trapped within these interstices. The slurry is then cast into a mold and cured into a solid propellant. This solid propellant contains randomly oriented metallic staple distributed uniformly throughout.

The propellant slurry employed in this invention is comprised of a casting solvent, densified nitrocellulose, metallic staple, and casting powder granules. Additionally these slurries can contain oxidizers, various fuels, stabilizers and ballistic modifiers. When these slurries contain additional ingredients such as oxidizers, the resulting propellant is referred to as composite modified double base (CMDB) propellant.

The casting solvents which can be employed in the process of this invention are comprised of an explosive liquid such as nitroglycerin, butane triol trinitrate, diglycol dinitrate, ethylene glycol dinitrate, and the like. These explosive liquids are mixed with one or more miscible deterrents, such as dibutyl phthalate, triacetin, dimethyl sebacate, dibutyl succinate, ethyl diphenyl phosphate, tributyl phosphate, and the like, to form a casting solvent mixture.

The densified nitrocellulose which is employed in the slurry can have a spherical, ellipsoidal or rounded shape, a particle size ranging from about 10 1. to about p. and a density range of from about 1.40 grams/cc. to about 1.60 grams/cc. The densified nitrocellulose is employed in the slurry in an amount of from about 5% to about 20% by weight based on the Weight of the slurry. The densified nitrocellulose supplements the nitrocellulose content on the surface of the casting powder granules, thereby providing sufiicient swelling in the interstices between casting powder granules during curing, so that the resulting propellant has satisfactory physical properties.

Metallic staple which is employed in the slurries can be prepared from aluminum, zirconium, tantalum, magnesium, silver, copper, titanium, alloys of these respective metals, and the like. Metallic staple is incorporated into the slurry preferably in the form of very thin strips. The size of the metallic staple which can be employed is dependent on the dimensions of the casting powder granule. The length of the metallic staple can be equal to, but is preferably less than the length of the casting powder granule. The width of the metallic staple can be equal to, but is preferably less than the diameter of the casting powder granule. The relationship between the dimensions of the casting powder granules and metallic staple heretofore described is important since the randomizing function of the casting powder granules is lost if the length and width of the staple exceeds the length and diameter of the granules. The metallic staple is incorporated into the slurry in an amount of up to about 5% by weight and generally from about 0.2% to about 5% by weight based on the weight of the densified slurry.

The casting powder granules which can be incorporated into the slurry heretofore described are prepared in the usual manner employed in smokeless powder production. These granules can be of single base, double base, or triple base formulation and can have various fuels, oxidizers, ballistic modifiers, stabilizers and other adjuvants incorporated therein as desired. These granules are sometimes referred to in the art as smokeless powder granules or base grain. The casting powder granules are admixed into the slurry in an amount of from about 15% to about 35% by weight based on the weight of the final slurry composition. When the percentage of casting powder in the slurry is below about 15%, there are insufficient casting powder granules present to efiiectively orient the metallic staple. When the percentage of casting powder in the slurry is greater than about 35%, the slurry is too viscous to be cast. In general the slurries can have a viscosity of about 10,000 centipoise to about 400,000 centipoise. The weight ratio of casting powder granules to metallic staple employed in the slurry can vary from about 5/1 to about 50/1. It is preferable, however, to have a weight ratio of about 20/ 1 or higher.

Oxidizers which can be admixed in the propellant slurry when preparing a composite modified double base propellant include ammonium perchlorate, cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX), and the like. Mixtures of two or more oxidizers can be employed if desired.

In preparing a propellant slurry of this invention for casting, it is generally preferred to prepare a first slurry comprised of a casting solvent, densified nitrocellulose, metallic staple, and various oxidizers, ballistic modifiers, stabilizers and combinations thereof if employed. This first slurry is mixed for about 30 minutes at ambient temperature. Casting powder granules are then admixed with this propellant slurry to form the final slurry composition. The final slurry composition contains the casting powder granules and is referred to hereinafter as the densified slurry. The densified slurry is subjected to vacuum and mixed for several minutes. Mixing of the densified slurry provides for random distribution of the casting powder granules and the magnetic staple throughout the slurry.

After addition of the casting powder granules to the first slurry to form the densified slurry, mixing should be limited to about ten minutes. Longer mixing times result in a substantial increase in the viscosity of the densified slurry due to attack of the nitrocellulose in the casting power granules by the casting solvent. The densified slurry is cast into suitable molds and cured, preferably at an elevated temperature of from about 120 F. to about 140 F. Suitable curing times range from about 48 hours to about 96 hours depending on the particular propellant formulation. The resulting solid propellant charge has metallic staple randomly oriented throughout. While the above procedure is preferred, various modifications in the order of addition of ingredients to the slurry can be made provided suflicient mixing of the slurry can be accomplished for random orientation of metallic staple prior to casting.

The following examples will further illustrate this invention. All parts and percentages are by weight unless otherwise specified. In these examples, the casting powder granules that are admixed with various propellant slurries to provide densified slurries have the composition listed in Table I. These casting powder granules are prepared in the form of right cylinders having a length of 0.050 inches, and a diameter of 0.050 inches.

TABLE I Casting powder granule composition Component: 7 Percent Nitrocellulose 30.0 Nitroglycerin 17.5 Ammonium perchlorate 50.0 Resorcinol 1.5 2-nitro-diphenylamine 1.0

EXAMPLE 1 A first propellant slurry having the composition listed in Table II, column A, is prepared by admixing a casting solvent comprised of nitroglycerin and triacetin, with ammonium perchlorate, aluminum staple, densified nitrocellulose, resorcinol and Z-nitro-diphenylamine. About 68.7 parts of this first propellant slurry is admixed with 31.3 parts of casting powder granules having the composition specified in Table I. The resulting densified slurry composition (Table II, column B)'is cast undera vacuum of about 10 mm./Hg into beakers and cured for four days at F. The cured solid propellant is cut up and the exposed propellant face of each cut carefully examined. The random orientation of the staple can be seen by visual inspection of the face of each propellant cut.

Strands of propellant are then cut from the propellant mass and burned in a strand burning apparatus. The pressure-versus-burning rate curve for the propellant strands is a smooth curve illustrating random orientation of the staple in the propellant. For comparison purposes, strands of a propellant containing metallic staple and prepared from propellant made by casting of casting powder granules containing metallic staple are burned in the strand burning apparatus. This propellant has the composition listed in Table II, column C. Both the propellant burning rate and slope of the pressure-versus-burning rate curve for the strands prepared by the slurry process of this invention and strands made from the conventionally cast propellant are substantially identical. considerably less metallic staple is incorporated into the propellant prepared by the slurry process of this invention.

Staple dimensions: 0.00045 inches thick, 0.008 inches thick, 0.050 inches long.

EXAMPLES 2-8 Propellant slurries are prepared following the method set forth in Example 1 and employing casting powder granules having the composition specified in Table I. The resulting densified slurries are cured at about 140 F. into propellants having metallic staple randomly oriented throughout. Data on each of these slurries and on the cured propellant are given in Table III.

TABLE III Example number 2 3 4 5 6 7 8 Casting powder/staple ratio 6.5 6.5 8.7 12.5 25.0 20.0 25.0 Densified slurry composition:

Densified nitrocellulose 6.7 2.0 2.0 6.0 6.0 6.0 Casting solvent.. 33.4 38.3 38.6 36.5 36.5 36.5 Resorcinol 0.7 0.5 0.4 0.5 0.5 0.5 Ammonium per- 26.1 26.1 30.0 30.0 31.0 36.0 p 4.4 4.4 3.0 2.0 1.0 1.0 1.0 Casting powder. 28.7 28.7 26.0 25.0 25.0 20.0 25.0 Densified slurry conamen" e Cured gra1ncond1t10n OK OK OK OK OK Cured grain density (lb.in. .0616 .0613 .0612 .0611. .0609 .0612 Ba11istiescured propellant:

Burning rate (inches/second) at 2,000 psi 3.6 3.5 3.7 Pressure/burning rate slope 0.66 0.69 0.63

Aluminum, .005 inches thick, .002 inches wide, .050 inches long. b Thick. 0 Very thick. Fluid. 6 Soft and gummy.

Examples 2 and 3, Table III, illustrate that the densified slurry mix is thick and thus is difiicult to cast with casting powder/staple ratios of 6.5/1.0. As the casting powder to slurry ratio is increased as shown in Example 5 t0 12.5/1.0, the densified slurry becomes more fluid and is easier to cast.

Examples 3 and 4, Table III, illustrate the efiect of an insufiicient amount of densified nitrocellulose in the densified slurry. The cured grains of Examples 3 and 4 are soft and gummy and do not have satisfactory physical properties.

Examples 6, 7 and 8, Table III, illustrate the high burning rates that are achieved with small amounts of randomly oriented metallic staple in the cured propellant charge. The pressure-versus-burning rate slopes plotted from data from strand burning are smooth, illustrating random orientation of the metallic staple in the cured propellant charge.

The process of this invention is an economical process to make fast burning rate propellants. The process offers flexibility in obtaining a desired burning rate since staple concentration, size, and metal type can be readily changed while employing a fixed composition of casting powder. Another advantage of the process of this invention is that there is essentially no attrition of metallic staple during mixing. This is a problem encountered with preparation of smokeless powder containing staple.

As will be evident to those skilled in the art, various modifications can be made or followed in light of the foregoing disclosure and discussion, without departing from the spirit or scope of the disclosure or from the scope of the claims.

What I claim and desire to protect by Letters Patent is:

1. A method for preparing a solid propellant charge, having metallic staple randomly oriented throughout, by a slurry-casting process, said method comprising:

(a) preparing a propellant slurry comprising a casting solvent, densified nitrocellulose, metallic staple and smokeless powder granules of the single base, double base or triple base type, said smokeless powder granules being present in the slurry in an amount sufiicient to provide interstices between the granules for trapping the metallic staple,

(b) admixing said slurry whereby the metallic staple is trapped within the interstices between the smokeless powder granules,

(c) casting the slurry, and

(d) curing the slurry.

2. The method of claim 1 wherein the length and diameter of the smokeless powder granules are greater than the length and the width of the metallic staple.

3. The method of claim 2 wherein the propellant slurry contains from about 15% to about 35% of smokeless powder granules.

4. The method of claim 3 wherein the weight ratio of smokeless powder granules to metallic staple is from about 5/ 1 to about 50/1.

5. The method of claim 4 wherein the propellant slurry contains from about 5% to about 20% densified nitrocellulose.

6. The method of claim 5 wherein the metallic staple is aluminum.

7. The method of claim 5 wherein the metallic staple is zirconium.

8. A method for preparing a composite modified double base propellant charge, having metallic staple randomly oriented throughout, by a slurry casting process, said method comprising:

-( a) preparing a first propellant slurry comprising casting solvent, an oxidizer selected from the group consisting of ammonium perchlorate, cyclotrimethylene trinitramine, cyclotetramethylene tetranitramine and mixtures thereof, densified nitrocellulose and metallic staple,

(b) admixing smokeless powder granules of the single base, double base or triple base type, to the first propellant slurry to provide a densified slurry, said granules being added in an amount sufficient to provide interstices between the granules for trapping the metallic staple,

(c) admixing the densified slurry whereby the metallic staple is trapped within the interstices between the smokeless powder granules,

(d) casting the densified slurry, and

(e) curring the slurry.

9. The method of claim 8 wherein the oxidizer is ammonium perchlorate, and the casting solvent is comprised of nitroglycerin.

10. The method of claim 9 wherein the densified slurry 7 contains from about 15% to about 35% of smokeless powder granules, and from about 5% to about 20% of densified nitrocellulose.

11. The method of claim 10 wherein the metallic staple is aluminum and the weight ratio of smokeless powder gganlules to metallic staple is from about 5/1 to about References Cited 

